On the side of signal transmission, there is a demand for checking a video signal through a video monitor at the same time as transmitting the video signal, such as a television signal photographed by a camera. On the other hand, on the side of signal reception, there is a demand for performing various processes concurrently while checking the transmitted video signal through the video monitor. To satisfy these demands, it can be considered that a signal is divided into a plurality of signals and supplied to the devices, using a dividing device. In this configuration, a disadvantage is that the overall size of the configuration becomes large and that it lacks mobility. The device which has received the signal is equipped with a function (a loop through function) for looping through the received signal and externally outputting it.
As an example, as illustrated in FIG. 7, a conventional analog television monitor includes a loop through circuit configured using two connectors, inside the monitor, to realize a loop through function. That is, a loop through circuit 200 is configured with a connector 201, having a port 21 and a terminal 22, and a connector 202 having a port 24 and a terminal 23. It has a configuration in which the terminal 22 of the connector 201 and the terminal 23 of the connector 202 are shorted.
The loop through circuit 200 with this configuration introduces an external signal P21 input from the port 21 of the connector 201, into the device through the terminal 22 as a signal 25. The circuit 200 outputs a signal P24 with loop-through output from the port 24 through the terminal 23 of the connector 202, and supplies the signal to the device through the transmission line. The signal output from the terminal 25 of the loop through circuit 200 is supplied to a signal processing circuit 203 providing a high impedance input and being provided inside the device, and is introduced into the device for processing.
FIG. 8 illustrates an example of transmitting a signal to an “n” number of devices using this loop through circuit 200. The signal from an external signal source 20 is supplied to a port 21-1 of the loop through circuit 200-1 inside the device of the first stage, introduced into the device from a terminal 25-1, and also output from a port 24-1. The signal of this port 24-1 is input to a port 21-2 of the loop through circuit 200-2 of the device of the second stage, introduced into the device from a terminal 25-2, and output from a port 24-2. In this manner, the signal is sequentially transmitted, and the output of the port 24-n of the device of the “n”-th stage (as the final stage) is terminated by a terminating resistor 206. The signals introduced into the device from the terminals 25-1, 25-2, . . . 25-n of the stages are supplied to signal processing circuits 203-1, 203-2, . . . 203-n inside the devices. These signal processing circuits 203-1, 203-2, . . . 203-n provide a high impedance input, not to have an effect on the signal.
The loop through configuration of FIG. 8 can be expressed using an equivalent circuit of FIG. 9. That is, the signal from the external signal source 20 is transmitted to the terminating resistor 206 through the transmission line, and tapped out to the “n” number of devices, having a high impedance input and arranged in the middle of the transmission line.
However, the conventional method has a problem that the characteristic deteriorates due to an increase in reflection loss, because the recent digital television signal is a high speed/wide band pulse signal as compared with a conventional analog television signal. That is, according to the method using the loop through configuration of FIG. 8 to be an equivalent circuit of FIG. 9, the tap-out path from the middle of the transmission line will be a so-called stub. If the length of the stub is not negligible with respect to the signal wavelength, a negative effect is brought to the signal due to the reflection at the stub. For example, even if the length of the stub is negligible with respect to the analog signal of approximately 10 MHz, the length is not negligible with respect to a high speed/wide band pulse signal of several 100 MHz to several GHz, thus remarkably deteriorating the transmission characteristic. That is, as the signal bandwidth becomes wide, a relative length of the stub becomes longer as compared with the signal wavelength, resulting in deterioration of the transmission characteristic.
The present inventor has proposed an active connector (Patent Literature 1) which includes an equalizer circuit (for example, a cable equalizer), a buffer circuit (for example, cable driver) and the like in the connector base part, as a connector device that can be used easily by anybody without any special know-how, while obtaining a predetermined characteristic for the recent digital television signal. The above-described loop through circuit can be configured using this active connector. An example of this is illustrated in FIG. 10.
In FIG. 10, “201” identifies an active connector including a cable equalizer, and “202” identifies an active connector including a cable driver. “204” identifies a dividing circuit, and “203” identifies a signal processing circuit inside the device. The external signal P21 is supplied to the port 21 of the active connector 201, and output from the terminal 22 of the active connector 201. The output signal form the terminal 22 of the active connector 201 is supplied to the dividing circuit 204 arranged inside the device. An output of the dividing circuit 204 is supplied to the signal processing circuit 203 inside the device, while the other output thereof is supplied to the terminal 23 of the active connector 202 having the cable driver. Then, the signal P24 is output with loop-through output from the port 24 of the active connector 202.
As described in this example, when the conventional active connector is used, the dividing circuit 204 is necessarily arranged inside the device, in addition to the active connectors 201 and 202. However, because the dividing circuit 204 is provided on the outside of the connectors 201 and 202, the dividing circuit 204 and the connectors 201 and 202 are arranged consequently at a large physical distance therebetween. Thus, a problem is that there is an effect of mismatching between these active connectors 201 and 202 and the dividing circuit 204. For example, in the case of a signal having a transmission bandwidth up to 3 GHz, the wavelength in free space is approximately 100 mm, while the wavelength in a substrate whose relative permittivity is 4 to 5, like a glass epoxy substrate, is equal to or shorter than 50 mm. This implies that the distance of 5 mm on a substrate, for example, causes a phase shift of approximately 40°. That is, when the reflection occurs due to the impedance mismatching, the waveform is remarkably distorted.
To prevent or reduce this distortion, it is necessary to improve the accuracy of the impedance matching. However, parasitic elements in a high frequency circuit, which cause the mismatching, depend greatly on the wiring pattern or the circuit layout and also the experience of circuit designers or their know-how. In particular, the dividing circuit differs from a simple two-terminal circuit or four-terminal circuit, and is a circuit network of six terminals or more. Thus, it is very difficult to design the circuit excluding the effect of the mismatching. That is, even if the loop through circuit is configured using the active connector proposed by the present inventor, there is a disadvantage that the signal transmission characteristic remarkably depends on the wiring pattern or circuit layout of the dividing circuit, because the dividing circuit as terminal load is a six-terminal circuit network instead of a two-terminal circuit which the conventional active connector assumes.